Conventionally, a muffling piece has been employed to reduce noise in various places. For example, a muffling piece is sometimes placed in every position where noise reduction is needed such as an air-conditioning duct, an electric appliance, or a building. In general, the larger the surface area of the muffling piece is, the higher probability there is that the noise may hit the muffling piece. Therefore, when measures to increase the size of the surface area of the muffling piece are taken, such as installation of an additional muffling piece or an enlargement of the muffling piece, noise reduction performance may be improved.
Such muffling pieces are discussed in Japanese Laid-open Patent Publication No. 06-58151 and Japanese Laid-open Patent Publication No. 2005-30308, for example.
As described above, when the size of the surface area of the muffling piece is increased, the noise reduction performance may be improved. However, the measures such as the installation of the additional muffling piece may undesirably cause an increase in ventilation resistance, pressure loss, or the like in some places. Also, the noise may be undesirably increased because of the increase in ventilation resistance or pressure loss, possibly causing a decrease in the noise reduction performance. In addition, the installation of the additional muffling piece may also cause an increase in the cost of muffling equipment. The muffling pieces discussed in both the Japanese Laid-open Patent Publication No. 06-58151 and the Japanese Laid-open Patent Publication No. 2005-30308 muffle sounds corresponding to frequencies of the noise.